<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <meta http-equiv="X-UA-Compatible" content="ie=edge">
    <title>Document</title>
    <style>
        #web{
            width: 100%;
            background: skyblue;
            position: relative;
        }
        .content{
            position: relative;
            width: 70%;
            margin: 0 auto;
            background: white;
            z-index: 20;
        }
        .bottom{
            position: absolute;
            bottom:0;
            z-index: 10;
            height: 50px;
            background: tomato;
            width: 100%;
        }
    </style>
</head>
<body>
    <div id="web">
        <div class="content">
            <h1>主体</h1>
            <h1>主体</h1>
            <h1>主体</h1>
            <h1>主体</h1>
        </div>
        <div class="bottom">
                
        </div>
    </div>
</body>
</html>